Outline for Today CSE 142. Programming a Teller Machine. CSE142 Wi03 I-1. ATM Algorithm for Dispensing Money

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1 CSE 142 Outline for Today Iteration repeating operations Iteration in Java while statement Shorthand for definite (counting) iterations for statement Nested loops Iteration Introduction to Loops 1/10/2003 (c) , University of Washington I-1 1/10/2003 (c) , University of Washington I-2 Programming a Teller Machine Suppose you are working on the code for a automated teller machine (ATM). Your code should give out the right number of bills when the user withdraws money. The ATM contains $20 and $5 bills. Problem: Hand out the right number of $20 and $5 bills to make up d dollars. Assume that d is a multiple of $5. Best solution would use as many $20s as possible Design an algorithm for this with your neighbors ATM Algorithm for Dispensing Money Design your solution(s) here 1/10/2003 (c) , University of Washington I-3 1/10/2003 (c) , University of Washington I-4 CSE142 Wi03 I-1

2 Additional notes ATM Algorithm Iteration/Repetition The ATM cash algorithm is an example of an iteration or repetition repeatedly perform some operation A few more examples Bake the roast; keep checking the internal temperature until it reaches 220 degrees While there are still donuts in the box, eat one Lather, rinse, repeat Simulations/games science, entertainment Repeatedly update actions of objects in the simulation Video display frames repeatedly Practically all interesting programs contain loops 1/10/2003 (c) , University of Washington I-5 1/10/2003 (c) , University of Washington I-6 Grow Your Money... On your birthday, you spend $5000 to buy a Certificate of Deposit which earns 3.5% a year on its balance. There is a $10 service fee deducted from the balance each year. How much money will you have at the end of 5 years? Work with your neighbor to design an algorithm to solve the problem. Write down a brief description of your algorithm (without using Java code). Iteration in Java: while Syntax Basic form while statement while ( condition ) { Terminology condition is sometimes called the loop condition is often called the loop body 1/10/2003 (c) , University of Washington I-7 1/10/2003 (c) , University of Washington I-8 CSE142 Wi03 I-2

3 Iteration in Java Meaning of while ( condition ) { Repeatedly do the following: Evaluate the condition If the condition is false, the loop terminates execution continues with the statement following the loop body (after ) Execute the and repeat Note: condition is only reevaluated after finishing the complete execution of the loop body not concurrently as loop body statements are executed Flow Chart Another way to visualize loop execution 1/10/2003 (c) , University of Washington I-9 1/10/2003 (c) , University of Washington I-10 Exercise Write Numbers and Squares Suppose we want to write a table of numbers and their squares for the numbers 1 to 5 Brute force ( + used to combine strings) System.out.println(1 + squared = + 1*1); System.out.println(2 + squared = + 2*2); System.out.println(3 + squared = + 3*3); System.out.println(4 + squared = + 4*4); System.out.println(5 + squared = + 5*5); How could we improve this? What We re Really Trying to Do We really want to repeatedly execute System.out.println(k + squared = + k*k); with k taking on the values 1 through 5 on successive repetitions Solution (?) k = 1; while (k <= 5) { System.out.println(k + squared = + k*k); Does this work? How can we tell? 1/10/2003 (c) , University of Washington I-11 1/10/2003 (c) , University of Washington I-12 CSE142 Wi03 I-3

4 Tracing Loops You can desk-check code by hand simulating the steps the computer performs Desk-checking a loop involves simulating each iteration of the loop Check: Exercise In arithmetic, n! (read as n factorial ) is defined to be 1 * 2 * 3 * 4 * * (n-1) * n Exercise: write a loop to compute 7! and check it Hint(?): try writing this out by hand, then figure out what statements can be repeated while some values in them change 1/10/2003 (c) , University of Washington I-13 1/10/2003 (c) , University of Washington I-14 Loop to Calculate 7! Check: Trace Your code here 1/10/2003 (c) , University of Washington I-15 1/10/2003 (c) , University of Washington I-16 CSE142 Wi03 I-4

5 Counting Loops For Statement The loops we ve seen so far all execute a definite number of times with some variable taking on a sequence of values Java, like most other languages, provides a special statement to make this convenient the for statement for (initialization; condition; update) { Counting Loops Example: Print the numbers 1 through 100 With a while loop int k = 1; while (k <= 100) { System.out.println(k); k = k + 1; With a for loop for (int k = 1; k <= 10; k = k + 1) { System.out.println(k); These mean exactly the same thing Style point: Java programmers would normally use for instead of while for a counting loop. 1/10/2003 (c) , University of Washington I-17 1/10/2003 (c) , University of Washington I-18 For Loops and While Loops A for statement is a convenient shorthand for an equivalent while statement for (initialization; condition; update) { has (for our purposes) exactly the same meaning as initialization; while ( condition ) { update Note that the update executes after the loop body For Statement Flow Chart 1/10/2003 (c) , University of Washington I-19 1/10/2003 (c) , University of Washington I-20 CSE142 Wi03 I-5

6 Factorial as a Method A calculation like factorial is a logically coherent operation. It makes sense to package it as a method. Complete the implementation below using a for statement /** Return the value n! */ public int factorial(int n) { Shortcut In loops like for (int k = 1; k <= 10; k = k + 1) { the update k = k + 1 is so common that Java provides a shorthand way to write it: k++ Equivalent loop for (int k = 1; k <= 10; k++) { 1/10/2003 (c) , University of Washington I-21 1/10/2003 (c) , University of Washington I-22 Double Your Money Problem: Suppose you have invested $1000 at 3% annual interest (meaning that each year, 3% of the present value of the investment is added to it). How many years will it take to double the original investment? Analysis: repeatedly increase the investment value by 3% until it reaches $2000. Count how many times this has to be done. A Non-Counting Iteration In this problem, the operation needs to be repeated until something happens (value >= $2000) We don t know how long this will take This is an indefinite iteration the number of repetitions needed is not known in advance A while loop is appropriate here 1/10/2003 (c) , University of Washington I-23 1/10/2003 (c) , University of Washington I-24 CSE142 Wi03 I-6

7 Your Code Here Double Your Money Nested Loops How would you print the following figure? * * * * * * * * * * * * * * * Useful information: System.out.print( * ); will print a single * System.out.println( * ); will print a single *, then move to the beginning of the next output line How would your answer need to be changed if we changed the number of rows or columns? 1/10/2003 (c) , University of Washington I-25 1/10/2003 (c) , University of Washington I-26 Analysis Solution 1/10/2003 (c) , University of Washington I-27 1/10/2003 (c) , University of Washington I-28 CSE142 Wi03 I-7

8 Check Trace the Code Another Problem: A Multiplication Table How would we print the following table? Analysis 1/10/2003 (c) , University of Washington I-29 1/10/2003 (c) , University of Washington I-30 Your Solution Here Multiplication Table Code Trace your code here Multiplication Table Check 1/10/2003 (c) , University of Washington I-31 1/10/2003 (c) , University of Washington I-32 CSE142 Wi03 I-8

9 Summary All interesting programs contain iteration repetition of statements Basic loop while statement Method of choice for indefinite iterations Normal shorthand for definite iterations for statement Nested loops 1/10/2003 (c) , University of Washington I-33 CSE142 Wi03 I-9